The present invention relates generally to electro-optical systems, and more specifically, to systems that require line of sight pointing and stabilization.
Precision optical instruments mounted on mobile platforms need to point accurately and reject disturbances to the line of sight. At sub-microradian levels of accuracy, significant disturbances to the line of sight originate within the optical system itself. To sample and correct for these errors requires the creation of an optical reference in inertial space, similar to a fixed star. Such a reference allows detection and correction of internal jitter sources along the optical train, and this “virtual star” supports both increased pointing accuracy and line of sight stabilization requirements.
A previously developed system approach developed by the assignee of the present invention is disclosed in U.S. patent application Ser. No. 09/828,483, entitled “Optical Line-of-Sight Pointing and Stabilization System.” That system includes a set of primary optics and relay optics that can be used to receive an image or transmit a laser. An inertially stable reference laser beam is injected into the primary optics and transmit the same beam path as the received image or transmitted laser. A jitter rejection mirror is located in the path of the optical system near the point at which an image is viewed or at which a laser originates in a laser pointing system. The jitter rejection mirror is displaced in a direction to oppose any change in the apparent position of the inertially stable reference laser beam.
For the purpose of jitter suppression, the Optical Inertial Reference Unit serves as an inertially stable platform on which a reference laser is mounted. This means that the reference laser beam leaves the Optical Inertial Reference Unit platform with minimal jitter induced by the host structure. Therefore it behaves as a “virtual star.” The reference laser beam samples the optical train jitter between the optical inertial reference unit and the auto-alignment sensor. The auto-alignment system uses an optical position sensor to sense the relative angle of the reference laser with respect to its boresight position. The sensed position is sampled, scaled, and compensated to send torque commands to the fast steering mirror. This digital alignment servo loop continuously seeks to hold the light of the virtual star on the null position of the alignment position sensor. This greatly attenuates the jitter in the optical train. Since the instrument beam paths (imaging and propagating) are common to that of reference laser, their jitter is also reduced.
The Optical Inertial Reference Unit platform also provides an inertial pointing reference. Once the sensors are initialized and calibrated in inertial space, they maintain knowledge of their attitude in that space. The Optical Inertial Reference Unit reference beam can then be commanded to point in any direction in that space. With the IRU platform mounted on the primary mirror, the angle between the IRU stable platform and its base can be used as an error signal to drive the primary mirror gimbals. This moves the line of sight of the optical system to desired pointing.
A previously developed inertial reference unit approach developed by the assignee of the present invention is disclosed in U.S. patent application Ser. No. 10/173,627 entitled “Optical Inertial Reference Generator.” That approach described a stabilized laser source as an optical line of sight reference.
The assignee of the present invention has reduced the Optical Inertial Reference Generator to the practice for two applications that differ primarily in performance. The first reduction to practice is an Optical Inertial Reference Unit for a NASA scientific program with a performance goal of 150 nanoradian root mean square residual beam jitter. The second reduction to practice, for a defense application, has a far more stringent 20 nanoradian residual beam jitter performance goal. The present invention embodies key advances in the state of the art to enable these compact high-performance applications.
Historically, it has been customary to mount a low-frequency (DC) sensor, for example a gyroscope, on a stabilized platform to provide feedback. This is undesirable for several reasons: (1) current technologies for high-accuracy low-drift-rate DC sensors result in large and massive sensors; these large heavy sensors necessitate increased size and weight of the platform which limits control bandwidth, (2) high-accuracy low-drift-rate DC sensors attached to the platform introduce asymmetries to the moments of inertia which further complicate the controls, (3) since high-performance DC sensors are expensive system components, it is costly and redundant to mount a DC sensor on the platform when another source of DC signals is already available within the system, and (4) some DC sensors impart reaction disturbances into the platform, thereby reducing stabilization performance or complicating the controls. The present invention removes the low-frequency DC sensor from the platform and implements an innovative Sensor Blending Kalman Filter to incorporate low-frequency signals from off-platform into the platform control loop. This innovation results in smaller size, lower weight, lower power, lower cost, more benign platform jitter, and better residual jitter performance.
In addition, the historic state of the art for stabilized platforms did not consider the use of redundant symmetrically arranged sensors and alignment of platform center of mass, center of rotation, and center of force application. The current invention embodies an innovative use of symmetry and redundant sensors to suppress unwanted moments, lower the drive requirements, and improve stabilization performance.
Two key components of the preferred embodiment of the present invention have been previously disclosed by the assignee of the present invention. First, a previously developed high-bandwidth angular rate sensor is disclosed in U.S. Pat. No. 5,067,351, entitled “Magnetohydrodynamic Angular Rate Sensor for Measuring Large Angular Rates.” Second, a previously developed high-precision linear displacement sensor is disclosed in U.S. Pat. No. 5,469,053, entitled “E/U Core Linear Variable Differential Transformer.” These two low-noise high-bandwidth sensors enable the preferred embodiment of the present invention to achieve very high optical reference beam stability.
It is an objective of the present invention to provide a compact optical reference unit with kilohertz bandwidth sub-microradian pointing and jitter control.